Magnetic and swept frequency systems, components and subsystems are disclosed in U.S. Pat. Nos. 3,810,147, 3,828,337, 3,863,244, 3,913,219, 3,938,044, 3,961,322, 3,967,161, 4,021,705, 4,117,466, 4,168,496, 4,243,980, 4,251,808, 4,260,990, 4,498,076, 4,567,473, and in Canadian Pat. No. 1005546, of the same inventive entity as herein, each being incorporated herein by reference. U.S. Pat. No. 3,961,322 discloses receiver circuitry operative to distinguish an interrogation signal in the form of an interrogation signal pulse train from environmental and other spurious signals. In response to the pulse height of the initial pulse of the received pulse train, the therein disclosed receiver circuitry starts a counter. The counter with each increment enables a sequential pulse discrimination module until it cycles through a predetermined time sequence that sequentially enables all of the pulse discrimination modules. The prior system, however, cannot be reset until after the entire cycle has been gone through, so that during this whole time, the prior system is locked-up, and is subjected to an undesirable failure of alarm situation. Moreover, each pulse discrimination module of the prior system is limited with respect to the capability that it is able to discriminate the characteristics of the pulses, such that if the pulses meet predetermined merely minimum duration conditions, the disclosed receiver circuitry produces an alarm, notwithstanding that the pulses may later exhibit totally unexpected characteristics typically representative of noise, so that the prior system is thereby subjected to an undesirable false alarm situation.